Apart from C++ and Octave language (m-files), Octave's sources include
files written in C, Fortran, M4, Perl, Unix shell, AWK, Texinfo, and
@TeX{}. There are not many rules to follow when using these other
languages; some of them are summarized below. In any case, the golden
rule is: if you modify a source file, try to follow any conventions you
can detect in the file or other similar files.

If you modify a Fortran file, you should stay within Fortran 77 with
common extensions like END DO. Currently, we want all sources to
be compilable with the f2c and g77 compilers, without special flags if
possible. This usually means that non-legacy compilers also accept the
sources.

The M4 macro language is mainly used for Autoconf configuration files.
You should follow normal M4 rules when contributing to these files.
Some M4 files come from external source, namely the
Autoconf archive.